1. Field of the Invention
The present invention relates to communications in computer networks. In particular, the invention relates to ranging methods in a data-over-cable system.
2. Description of the Related Art
Cable television networks such as those provided by Comcast Cable Communications, Inc., of Philadelphia, Pa., Cox Communications of Atlanta, Ga., Tele-Communications, Inc., of Englewood Colo., Time-Warner Cable, of Marietta Ga., Continental Cablevision, Inc., of Boston Mass., and others provide cable television services to a large number of subscribers over a large geographical area. The cable television networks typically are interconnected by cables such as coaxial cables or a Hybrid Fiber/Coaxial (“HFC”) cable system which have data rates of about 10 Mega-bits-per-second (“Mbps”) to about 30+ Mbps.
The Internet, a world-wide network of interconnected computers, provides multi-media content including audio, video, graphics and text that typically requires a large bandwidth for downloading and viewing. Most Internet Service Providers (“ISPs”) allow customers to connect to the Internet via a serial telephone line from a Public Switched Telephone Network (“PSTN”) at data rates including 14,400 bps, 28,800 bps, 33,600 bps, 56,000 bps and others that are much slower than the about 10 Mbps to about 30+Mbps available on a coaxial cable or HFC cable system on a cable television network.
With the explosive growth of the Internet, many customers have desired to use the larger bandwidth of a cable television network to connect to the Internet and other computer networks. Cable modems, such as those provided by 3Com Corporation, of Santa Clara, Calif., Motorola Corporation, of Arlington Heights, Ill., Hewlett-Packard Co., of Palo Alto, Calif., Bay Networks, of Santa Clara, Calif., Scientific-Atlanta, of Norcross, Ga., General Instruments, of Horsham, Pa., and others offer customers higher-speed connectivity to the Internet, an intranet, Local Area Networks (“LANs”) and other computer networks via cable television networks. These cable modems currently support a data connection to the Internet and other computer networks via a cable television network with a data rate of up to about 30+ Mbps, which is a much larger data rate than can be supported by a modem used over a serial telephone line.
Many cable television networks, however, provide only uni-directional cable systems, supporting only a “downstream” cable data path. A downstream data path is the flow of data to a customer from a cable system “headend”—a central location in the cable television network that is responsible for sending cable signals in the downstream direction. Traditionally, cable television networks provided one-way cable systems with a return “upstream” data path via a telephone network (i.e., a “telephony return”), such as a public switched telephone network provided by AT&T, GTE, Sprint, MCI and others. An upstream data path is the flow of data from the customer back to the cable system headend. A cable television system with an upstream connection to a telephony network is called a “data-over-cable system with telephony return.”
As the cable television network has developed, two-way cable systems without telephony returns, wherein both downstream and upstream flow occur on the cable network, have become increasingly prevalent. In a two-way cable system, customer premise equipment sends response data packets to the cable modem, which sends the data packets upstream via the cable television network to the cable modem termination system. The cable modem termination system sends the data packets to appropriate hosts on the data network and sends the response data packets back to the appropriate cable modem.
As a cable modem is initialized in a data-over-cable system, it registers with a cable modem termination system to allow the cable modem to receive data over a cable television connection from a data network (e.g., the Internet, a WAN or a LAN). The cable modem forwards configuration information it receives in a configuration file during initialization to the cable modem termination system as part of a registration request message. A cable modem also helps initialize and register any attached customer premise equipment with the cable modem termination system.
When a cable modem initializes on a cable television network, it uses a ranging process to determine the appropriate transmission parameters for its data transfer For a description of the ranging process, see DOCSIS Radio Frequency Interface Specification SP-RFIv1.1-I03-991105 (hereinafter “DOCSIS RFI Spec 1.1”), which is incorporated herein by reference. Under existing ranging procedures, the cable modem sends a ranging message at an initial maintenance opportunity. The initial maintenance opportunity is a contention opportunity under existing standards, and thus any cable modem attached to the network may be attempting to use the same opportunity. If two or more cable modems attempt to use the same initial maintenance opportunity, a collision occurs and the cable modem termination system does not receive any of the ranging messages. When the cable modems fail to receive a response from the cable modem termination system, the modems determine a random “backoff period” that the modem will wait before attempting to resend the ranging request.
As the number of cable modems using cable networks increases, this method will become increasingly inefficient. In order to accommodate a larger number of modems on the network, the modems may need to use larger backoff periods. Moreover, there is no gaurantee that another collision will not occur when the modem again transmits its ranging request. These limitations will lead to an increased number of ranging attempts that a cable modem must perform before accessing the cable modem termination system. The effects of these limitations will be especially pronounced when a downstream channel fails, and all the cable modems previously using that channel must re-initialize on another channel or on the same channel after it comes back online.
There is therefore a need in the art for an improved method of ranging.